The requirement that lead by phased out and the introduction of premium unleaded gasoline has created a strong demand for increased gasoline octane numbers. Conventional approaches such as increasing operating severity in reformers and fluid catalytic cracking units, or using octane catalysts and additives in fluid catalytic cracking units result in losses of gasoline yields. In addition, these approaches often increase the fuel gas yields in a refinery which may sometimes cause a reduction in refinery throughput and profitability.
Typical gasoline contains 2 to 5 liquid volume percent benzene, a chemical which has a high octane blending value, but is considered hazardous to human health and environment. The State of California, for example, has included benzene on its toxic chemical list, and the State of California Air Resources Board and the United States Environmental Protection Agency are considering regulations to limit the amount of benzene which may be present in gasoline to a level much lower than what is found in current gasoline. It is therefore highly desirable to remove benzene from gasoline. However, physically separating benzene from gasoline by distillation or extraction has the undesirable effect of decreasing both the octane rating and the volume of gasoline.
As an alternative, benzene and gasoline may be hydrogenated to a non-aromatic compound. This approach is also undesirable, because it requires a relatively high pressure operation and consumes hydrogen which is usually expensive in a refinery. Hydrogenation of benzene also reduces the octane rating of the gasoline.
To overcome these disadvantages, it has been found that benzene may be alkylated with resulting actual improvements in both octane and volume of gasoline produced. Co-pending U.S. Patent application Ser. No. 64,121, filed Oct. 28, 1988, discloses reacting a refinery stream with an olefin-containing stream in a distillation column reactor in the presence of an alkylation catalyst to thereby alkylate light aromatics, particularly benzene.
The chemical reactions involving alkylation of aromatics with olefins have been studied for a long time. For example, U.S. Pat. No. 2,860,173 discloses the use of a solid phosphoric acid (SPA) as a catalyst for the alkylation of benzene with propylene to produce cumene. U.S. Pat. No. 4,347,393 discloses the use of Freidel Crafts catalyst, especially aluminum chloride, for this reaction. More recently, certain rare earth modified zeolites and Mobil's HZSM-5 zeolite catalyst have been used to carry out this reaction. Examples may be found in the Journal Of Catalysis Volume 109, pages 212-216 (1988).
The alkylation of benzene with ethylene to produce ethylbenzene is a known commercial process. The Mobil/Badger ethylbenzene process produces high purity ethylbenzene in vapor phase with a multiple-bed reactor and a series of distillation columns. A description of the process using a dilute ethylene stream may be found in the Oil and Gas Journal, Volume 7, pages 58-61 (1977).
It is important to distinguish that while catalytic aromatic alkylation is known, it is subject to the unexpected and unpredictable vagaries of catalytic processes. For example, in U.S. Pat. No. 3,527,823 (Jones) there is disclosed the reaction of benzene and propylene over phosphoric acid catalyst in a fixed bed upflow reactor to produce cumene. While the benzenepropylene reaction was successful, the Jones process was not applicable to the reaction of benzene and ethylene (column 13, line 36). Poor yields of ethyl benzene were obtained by Jones. However, increased ethylene purity increased the conversion of ethylene (column 13, line 10) although the yield of ethyl benzene was still not satisfactory. In another U.S. Pat. No. 3,437,705, Jones discloses the alkylation of an aromatic compound with an olefin in an aromatic to olefin mol ratio of from 2:1 to 30:1. The process is characterized by the presence of an unreacted vapor diluent, such as propane, in the reaction zone. The total alkylation effluent is passed to a flash distillation zone where the unreacted diluent is separated. The process is purportedly applicable to a variety of reactions using feedstocks containing unreactive vapor diluents.
The concept of catalytic distillation, to the extent chemical reactions and distillation are carried out in the same vessel, is known. U.S. Pat. No. 3,629,478 discloses a method for separating linear olefins from tertiary olefins by feeding a mixture of alcohol, tertiary pentenes and linear pentenes to a distillation column reactor, atalytically reacting the tertiary pentenes with the alcohol by contacting them with heterogeneous atalyst located above the feed zone, and fractionating the ether from the linear pentene in the distillation column reactor. U.S. Pat. Nos. 3,634,534 and 3,634,535 disclose a method for separating a first chemical from a mixture of chemicals using two distillation column reactors in series. In the first distillation column reactor, the first chemical undergoes a reaction to form a second chemical which is easily fractionated from the mixture of chemicals. This second chemical is then fed to the second distillation column reactor, where the reaction is reversed and the first chemical is recovered by fractionation.
U.S. Pat. Nos. 4,232,177 and 4,307,254 disclose a method for conducting chemical reactions and fractionation of a reaction mixture comprising feeding reactants to a distillation column reactor into a feed zone and concurrently contacting the reactants with a fixed bed catalytic packing to carry out both the reaction and fractionate the reaction mixture. One example is the preparation of methyl tertiary butyl ether (MTBE) in high purity from a mixed feed stream of isobutene and ion exchange resin. U.S. Pat. No. 4,242,530 discloses a method for the separation of isobutene from a mixture comprising n-butene and isobutene by feeding a C.sub.4 stream to a distillation column reactor and contacting the stream with fixed bed acidic cation exchange resin to form diisobutene which passes to the bottom of the column, the n-butene being removed overhead. U.S. Pat. No. 4,624,748 discloses a novel catalyst system for use in a distillation column reactor which includes angularly-defined spaces within the reactor.
U.S. Pat. No. 4,849,569 (Smith) discloses a process for alkylating aromatic compounds by contacting the aromatic compound with a C.sub.2 to a C.sub.20 olefin in a distillation column reactor containing a fixed bed acidic catalyst comprising molecular sieves and cation exchange resins. The mol ratio of aromatic compounds to olefin is in the range of 2-100:1, since the greater the excess of aromatic compound the more selectivity is given to the desired product.
In spite of the art discussed, catalytic distillation reaction processes are not conventionally applied to complex hydrocarbon feedstocks and catalytic reactions thereof. It is important to distinguish that while such U.S. Pat. Nos. as 3,629,478 (Haunschild), 4,849,569 (Smith) and 4,471,154 (Franklin) disclosed the use of distillation reactors, these patents do not suggest the use of complex refinery streams as feedstocks for such distillation reaction processes. Refinery streams are complex when they contain many different chemical components in a boiling range. Conventional distillation reaction processes are limited to reactive feed streams each of which is relatively pure, in the sense that each is composed of chemical constituents having some physical and/or chemical similarity.
A paper entitled "Alkylation of FCC Off Gas Olefins with Aromatics Via Catalytic Distillation", I. E. Partin was presented at the National Petroleum Refineries Association Meeting, Mar. 22, 1988. This paper discloses a catalytic distillation process which alkylates the refiners light olefin gases such as ethylene and propylene, present in FCC and coker unit tail gas with light aromatics such as benzene and toluene, present in reformate to produce alkylated aromatics.
In the process as taught in this paper full range reformate is charged to the lower distillation section and the total FCC off gas stream charged beneath the catalyst section. The solid proprietary catalyst is secured within supports which form bundles for installation in the distillation tower. As olefins and aromatics proceed into the catalyst section and react, the heavier alkylated aromatics drop out into the lower fractionation zone and out the bottom of the tower with the heavy portion of the reformate. Light components, including light gases, proceed through the reactor and are stripped through the upper distillation section. Part of the unreacted benzene is recycled back to the tower to increase benzene conversion. Non-condensible gases go to fuel gas and light liquid is circulated back to the refinery gas plants or sent to gasoline blending.
The present process is applicable to the product streams from a number of refining processes, including fluid catalytic cracking (FCC), coking, and catalytic reforming, among others. Fluidized catalytic cracking (FCC) of heavy petroleum fractions is one of the major refining methods to convert crude or partially-refined petroleum oil to useful products, such as fuels for internal combustion engines and heating oils. A principal product of the FCC process is FCC gasoline, i.e., a liquid fraction boiling in the gasoline-range. FCC gasoline can contain a minor amount of benzene and other aromatics. The products may also include a mixture of hydrocarbon gases ranging from hydrogen, methane, ethylene, ethane, propylene, propane, to butylene, isobutane, butane, and heavier hydrocarbon gases. Various fractions of the gases are recovered in a vapor recovery unit.
While the details of a vapor recovery unit may vary, a typical arrangement involves first feeding the reactor effluent into a main fractionator. The fractionator overhead is compressed and fed into a de-ethanizer where the C.sub.2 and lighter gas entrained with some C.sub.3 's and C.sub.4 's is separated as an overhead product and fed into a sponge absorber. A lean sponge oil, typically a slip stream of heavy gasoline or light cycle oil, is used in the absorber to recover as much as possible the C.sub.3 + components in the de-ethanizer overhead. The rich sponge oil is usually returned to the main fractionator. Although it may still contain some C.sub.3 + components, the absorber overhead is usually called off-gas and is used as refinery fuel after some treating for sulfur removal. The de-ethanizer bottoms are fed into a de-propanizer where most of the propane/propylene gas is recovered as overhead.
Coking is a method to minimize refinery yields of residual fuel oil by severe thermal cracking of stocks such as vacuum residuals and thermal tars. It has been used to prepare coker gas oil streams suitable for feed to a catalytic cracker, to prepare hydrocracker feedstocks, to produce a high quality "needle coke" from stocks such as catalytic cracker heavy cycle oil, and to generate low BTU refinery fuel gas. Similar to atalytic cracking, coking produces a range of gas and liquid products which are separated in a distillation section. The lightest fraction which goes through a sponge oil absorber is usually called tail gas or off-gas and is used as refinery fuel gas.
Catalytic reforming is a method to convert low octane gasoline and naphtha streams into higher octane gasoline blending stock. The process typically increases the aromatic contents from 5%-10% in feed to 45%-60% in the liquid product, which is called "reformate". The benzene content makes up only from 2% to 10% of the reformate and is therefore a minor component of the reformate. The liquid products from a catalytic reformer are typically debutanized in a debutanizer which is sometimes called a stabilizer. The reformate is either sent directly to storage, or further separated to light reformate and heavy reformate. In some refineries, light aromatics such as benzene, toluene, and xylene are recovered as chemicals.
It would be advantageous if the minor amount of benzene in FCC gasoline and reformate could be alkylated to the maximum extent by the appropriate selection of reaction process and catalyst, using available olefin-containing refinery feedstocks.
The present invention overcomes the disadvantages of the prior art in that alkylation of benzene is carried out without loss of octane number or of volume of gasoline. Indeed, volume is somewhat increased. In accordance with a preferred embodiment of the present invention the alkylation portion of the process is carried out in a distillation reactor column. Preferably, the hydrogenation and isomerization portions of the process are also carried out in the distillation reactor column.